Mass spectrometry is a method of analyzing gas-phase ions generated from a particle molecular sample. The gas-phase ions are separated in electric and/or magnetic fields according to their mass-to-charge ratio. Analyzing molecular weights of samples using mass spectrometry consists mainly of three processes: generating gas phase ions, separating and analyzing the ions according to their mass-to-charge ratio and detecting the ions. The mass spectrometer is an instrument for implementing processes to measure the gas-phase mass ions or molecular ions in a vacuum chamber via ionizing the gas molecules and to measure the mass-to-charge ratio of the ions.
Formation of gas phase samples ions is an essential in a mass spectrometer. There are many ionization methods and related sources suitable for different kinds of samples. Ions may be generated by electron ionization (EI) in vacuum. EI is the most appropriate technique for relatively small (m/z<700) neutral organic molecules that can easily be promoted to the gas phase by heating without decomposition, (i.e. volatilization). Electron ionization is achieved through the interaction of an analyte with an energetic electron beam resulting in the loss of an electron from the analyte and the production of a radical cation. Electrons are produced by thermionic emission from a tungsten or rhenium filament. These electrons leave the filament surface and are accelerated towards the ion source chamber, which is held at a positive potential (equal to the accelerating voltage). The electrons acquire energy equal to the voltage between the filament and the source chamber, which typically is about 70 electron volts (70 eV).
In contrast to EI, most applications of chemical ionization (CI) produce ions by the relatively gentle process of proton transfer. The sample molecules are exposed to a large excess of ionized reagent gas. Transfer of a proton to a sample molecule M, from an ionized reagent gas such as methane in the form of CH5+, yields the [M+H]+ positive ion. Negative ions can also be produced under chemical ionization conditions. Transfer of a proton from M to other types of reagent gas or ions can leave [M−H]−, a negatively charged sample ion.
Corona discharge ionization is an electrical discharge characterized by a corona. Corona discharge ionization occurs when one of two electrodes placed in a gas (i.e. a discharge electrode) has a shape causing the electric field on its surface to be significantly greater than that between the electrodes. Corona discharges are usually created in gas held at or near atmospheric pressure. Corona discharge may be positive or negative according to the polarity of the voltage applied to the higher curvature electrode i.e. the discharge electrode. If the discharge electrode is positive with respect to the flat electrode, the discharge is a positive corona, if negative the discharge is a negative corona.
Desorption ionization is a term used to describe the process by which a molecule is both evaporated from a surface and ionized. Samples are desorbed and ionized by an impact process that involves bombardment of the sample with high velocity atoms, ions, fission fragments, or photons of relatively high energy. The impact deposits energy into the sample, either directly or via the matrix, and leads to both sample molecule transfer into the gas phase and ionization. Fast atom bombardment (FAB) involves impact of high velocity atoms on a sample dissolved in a liquid matrix. Secondary ion mass spectrometry (SIMS) involves impact of high velocity ions on a thin film of sample on a metal substrate or dissolved in a liquid matrix. Plasma desorption (PD) involves impact of nuclear fission fragments, e.g. from 252Cf, on a solid sample deposited on a metal foil. Matrix assisted laser desorption ionization (MALDI) involves impact of high energy photons on a sample embedded in a solid organic matrix.
In field desorption (FD), the sample is coated as a thin film onto a special filament placed within a very high intensity electric field. In this environment, ions created by field-induced removal of an electron from the molecule are extracted into the mass spectrometer.
Atmospheric pressure ionization (API) can generate sample ions from liquid solution in atmospheric pressure. Electrospray ionization (ESI) is a widely used method to produce gaseous ionized molecules desolvated or desorbed from a liquid solution by creating a fine spray of droplets in the presence of a strong electric field. The ESI source consists of a very fine needle and a series of skimmers. A sample solution is sprayed into the source chamber to form droplets. The droplets carry charge when the exit the capillary and as the solvent vaporizes the droplets disappear leaving highly charged analyte molecules. Electrospray ionization is the method of choice for proteins, oligonucleotides and metal complexes. However, the sample must be soluble in low boiling solvents (acetonitrile, MeOH, CH3Cl, water, etc.). Atmospheric pressure chemical ionization (APCI) is a relative of ESI. The ion source is similar to the ESI ion source. In addition to the electro hydrodynamic spraying process, a plasma is created by a corona-discharge needle at the end of the metal capillary. In this plasma, proton transfer reactions and possibly a small amount fragmentation can occur. Depending on the solvents, only quasi-molecular ions like [M+H]+, [M+Na]+ and M+ (in the case of aromatics), and/or fragments can be produced. Multiply charged molecules, as in ESI, are not observed. Atmospheric pressure photoionization (APPI) is a complement to ESI and APCI by expanding the range and classes of compounds that can be analyzed, including nonpolar molecules that are not easily ionized by ESI or APCI. The mechanism of photoionization —ejection of an electron following photon absorption by a molecule—is independent of the surrounding molecules, thereby reducing ion suppression effects.
In addition, plasma and glow discharge, thermal ionization and spark ionization are also used in mass spectrometry.
In conclusion, different phases and different kinds of molecular samples are ionized by different ionization methods. The same phase and same kind of molecular samples can be ionized by different ionization methods.
Mass analysis can also be performed using methods based on specific electric and/or magnetic field distributions or configurations. Several such configurations are described below:
A magnetic sector analyzer analyzes ion mass using a static magnetic field to disperse ions according to ion mass.
A quadrupole mass filter or quadrupole ion trap (QIT) or quadrupole linear ion trap (LIT) analyzer uses the stability or instability of ion trajectories in a dynamical electric RF field to separate ions according to their different m/z ratios. The quadrupole filter consists of four parallel metal rods. Both radio frequency (RF) voltages and direct current (DC) voltages with opposite polarities are applied across two pair of rods. Ions travel down the quadrupole in between the rods. Only ions of a certain m/z will reach the detector for a given ratio of RF and DC voltages: other ions have unstable oscillations and will collide with the rods. A quadrupole ion trap (QIT) mass analyzer is composed of a metal ring electrode and a pair of opposite metal end cap electrodes. The inner surfaces of the ring and two end cap electrodes are rotationally symmetric hyperboloids. Mass ion is trapped and then analyzed by so-called mass scanning methods. There are three different mass scanning methods to analyze the ion mass in commercial ion trap mass spectrometers: mass-selective instability scan, mass-selective scan by non-linear resonance, and mass-selective resonance scan by excitation frequency. The mass selective instability scan uses the stability boundary of the first stability region in Mathieu's stability diagram. During the mass scan, RF voltage applied the ring electrode is increased linearly. The working points of the mass ions are shifted across the stability border. The ions become unstable, oscillate in axial direction, and finally leave the ion trap through one of the end caps, one mass followed by another mass. The mass selective scan by nonlinear resonance uses the sharp amplitude growth in the ion oscillation due to nonlinear resonance conditions which arise in the ion trap caused by superposition of the quadrupole field with higher-order multipole fields. Such a nonlinear resonance condition occurs within the stability diagram. This method leads to particularly quick scanning to improve mass resolution. The mass-selective resonance scan is performed by applying an additional excitation frequency voltage between two cap electrodes. The ions are ejected from the ion trap by resonant dipolar excitation in the axial direction. The ions absorb energy from the dipole field and increase their oscillation amplitude at resonance. Ions leave the ion trap one mass followed by another mass, if the dipole field is sufficiently strong.
A Fourier Transformation Ion Cyclotron Resonance (FT-ICR) mass analyzer is based on the principle of ion cyclotron resonance. An ion placed in a magnetic field will move in a circular orbit at a frequency characteristic of its m/z value. Ions are excited to a coherent orbit using a pulse of radio frequency energy, and their image charge is detected on receiver plates as a time domain signal. Fourier transformation of the time domain signal results in the frequency domain FT-ICR signal which, on the basis of the inverse proportionality between frequency and m/z, can be converted to a mass spectrum.
A Time-of-flight (TOF) mass analyzer separates ions by m/z in a field-free region after accelerating ions to a constant kinetic energy. This acceleration results in any given ion having the same kinetic energy as any other ion. The velocity of the ion will however depend on the mass. The time that it subsequently takes for the particle to reach a detector at a known distance is measured. This time will depend on the mass of the particle (heavier particles reach lower speeds). From this time and the known experimental parameters one can find the mass of the particle.
The different mass analyzers have different features and advantages. For example, an ion trap analyzer has high sensitivity but medium mass resolving power, while TOF has a high mass accuracy and fast scan speed. ICR has ultra high mass resolving power and mass accuracy but is very expensive, which limits its wide application.
Tandem mass spectrometry, which is widely applied, involves at least two steps of mass selection or analysis, usually with some form of fragmentation in between. Coupling two stages of mass analysis (MS/MS) can be very useful in identifying compounds in complex mixtures and in determining structures of unknown substances. In product ion scanning, the most frequently used MS/MS mode, product ion spectra of ions of any chosen m/z value represented in the conventional mass spectrum are generated. From a mixture of ions in the source region or collected in an ion trap, ions of a particular m/z value are selected in the first stage of mass analysis. These “parent” or “precursor” ions are fragmented and then the product ions resulting from the fragmentation are analyzed in a second stage of mass analysis. If the sample is a mixture and soft ionization is used to produce, for example, predominantly [M+H]+ ions, then the second stage of MS can be used to obtain an identifying mass spectrum for each component in the mixture. For sector, quadrupole and time-of-flight instruments, each stage of mass analysis requires a separate mass analyzer.
A triple quadrupole mass spectrometer uses three quadrupole/multipole devices. The first quadrupole mass analyzer is used for parent ion selection, the second multipole collision cell is used for fragmentation and the third quadrupole is used for analyzing the fragmentation (daughter) ions. The quadrupole/TOF hybrid mass spectrometer, or Q-TOF, replaces the third quadrupole in triple quadrupole with TOF analyzer to give higher resolution and better mass accuracy. For quadrupole ion trap or ICR mass spectrometers, the MS/MS experiment can be conducted sequentially in time within a single mass analyzer. Ions can be selectively isolated, excited and fragmented, and analyzed sequentially in the same device. In addition, hybrid mass spectrometers may include a quadrupole linear ion trap combined with quadrupole ion trap (q-QIT), a quadrupole linear ion trap with FT-ICR, or an quadrupole ion trap with time-of-flight (QIT-TOF).
Several methods of fragmenting molecules for tandem mass spectrometry exist including collision-induced dissociation (CID), electron capture dissociation (ECD), Infrared multiphoton dissociation (IRMPD) and blackbody infrared radiative dissociation (BIRD). Collision-induced dissociation (CID), which is referred to by some as collisionally activated dissociation (CAD), is a mechanism by which molecular ions are fragemented in the gas phase. The molecular ions are usually accelerated by some electrical potential to high kinetic energy and then allowed to collide with neutral gas molecules (often helium, hydrogen or argon). In the collision, some of the kinetic energy is converted into internal energy, which results in bond breakage and the fragmentation of the molecular ion into smaller fragments. Electron capture dissociation (ECD) involves the introduction of low energy electrons to trapped gas phase ions. In infrared multiphoton dissociation (IRMPD), an infrared laser is directed through a window into the vacuum chamber of the mass spectrometer containing the ions. The mechanism of fragmentation involves the absorption by a given ion of multiple infrared photons. The parent ion becomes excited into more energetic vibrational states until a bond(s) is broken resulting in gas phase fragments of the parent ion. Blackbody infrared radiative dissociation (BIRD) uses the light from black body radiation to thermally (vibrationally) excite the ions until a bond breaks. This is very similar to infrared multiphoton dissociation with the exception of the source of radiation.
In a mass spectrometer, once ions generated by an ionization source, they must be transported by an interface device to the mass analyzer through a transfer region. This interface device may be an ion optical system such as a Radio frequency (RF) linear multipole ion guide. A RF linear multipole (quadrupole, hexapole, octopole, and so on) ion guide consists of four parallel metal rods (quadrupole) or six rods (hexapole) or eight rods (octopole) and is supplied with RF voltages with a typical RF frequency at 1-5 MHz. The ion guide is often used in coupling an elevated pressure ionization source, most an API source, such as ESI, to a mass analyzer operated in a vacuum of about 10−4 torr or higher. The use of linear multipole ion guides has been shown to be an effective means of transporting ions through vacuum. U.S. Pat. No. 4,963,736 (1990) described the use of an RF-only quadrupole ion guide to transport ions from an API source to a mass analyzer. U.S. Pat. No. 5,652,427 (1997) describes the use RF linear multiple ion guides to transfer ions from one pressure region to another in a differentially pumped system.
A linear multipole ion guide is easily converted into a linear ion trap by applying a static DC potential to electrodes at the entrance and the exit of the multipole ion guide device. Ions are then confined radially by a two-dimensional (2D) RF field, and axially by static DC potentials. In contrast to a three-dimensional (3D) ion trap, ions are not confined axially by RF potentials in a linear ion trap. A linear ion trap has a high acceptance since there is no RF quadrupole field along the z-axis. Ions admitted into a pressurized linear quadrupole undergo a series of momentum dissipating collisions effectively reducing axial energy prior to encountering the end of electrodes, thereby enhancing trapping efficiency. A larger volume of the pressurized linear ion trap relative to the 3D device also means that more ions can be trapped. Radial containment of ions within a linear ion trap focuses ions to a line, while the 3D ion trap tends to focus the trapped ions to a point. It has been recognized that ions can be trapped in a linear ion trap and mass selectively ejected in a direction perpendicular to the central axis of the trap via radial excitation techniques, or mass selective axially ejected in the presence of an auxiliary quadrupole field.
The linear ion trap has axially combined in serial with a 3D ion trap, a time-of-flight (TOF) or FT-ICR mass analyzer. Combining a linear ion trap with a 3D trap can help to overcome the limitation of poor duty-cycle (transmission efficiency) and space charge effects. Coupling a linear multipole trap to a 3D ion trap was been described in U.S. Pat. No 5,179,278. To improve the duty-cycle, ions are accumulated in the linear ion trap, whereas the 3D trap performs other functions such as CID or mass analysis.
Usually, the linear multipole ion guide or linear ion trap combines a single ion source to a single mass analyzer because it has two interfaces, entrance and exit, in the z-axis only. The present invention discloses a six-electrodes ion trap device and operating method, which can generate six “z-axis” interfaces in three-dimensional XYZ space and can be alternated from an axis to another. Ions can be injected in different directions and transferred to different directions for specific purposes. The design allows combination of multiple ion sources and mass analyzers into a single instrument.
Langmuir et al. first reported a six-electrodes ion trap in the early 60s. The ion trap was constructed with a specific six plane sheets of metal and mounted parallel to the faces of a cube. (See e.g., R. F. Wuerker, H. M. Goldenberg and R. V. Langmuir, J. Appl. Phys., 30, 44, 1959). In a later model by Haught and Polk, this sheet structure was replaced by a set of six annuli. (See e.g., A. F. Haught and D. H. Polk, Phys. Fluids, 9, 2049, 1966). These early six-electrode ion traps with simple surfaces generated very poor quadrupole field and have low trapping efficiency. All previous works of six-electrodes ion traps with the specific surface shapes were involved with ion storage or mass analyzing. Until the present invention, a six-electrode ion trap has not been used as a device to combine multiple ion sources and mass analyzers into a single instrument.